Determining the frequency of a periodic electrical signal is a common task performed in electronic systems. In the field of radio communications, for example, it is often necessary to determine the frequency of a communication signal in real-time.
Traditionally, analog circuitry has been used to determine the frequency of electrical signals. However, due to the complexity and unreliability of analog circuitry, and the trend toward all-digital radio systems, various digital frequency determining implementations have been proposed. U.S. Pat. No. 6,219,394 (‘the '394 patent’) discloses, for example, a frequency-to-digital converter (FDC) that operates to digitally determine the instantaneous frequency of a periodic digital signal.
FIG. 1 illustrates the two primary components of an FDC 100, similar to that which is disclosed in the '394 patent. As shown, the FDC 100 comprises a frequency sampling circuit 102 and a digital filter 104. The frequency sampling circuit 102 is configured to receive an input signal x(t) (or ‘test’ signal) having a frequency fx that is to be determined by the FDC 100. The sampling circuit 102 samples the input signal x(t) at a known sampling rate fS provided by a sampling clock s(t), and generates a stream of digital bits representing the ratio of fx to fS. The digital filter 104 is configured to receive the stream of digital bits, and, based on the pattern of logic ‘1s’ and ‘0s’ in the stream, operates to recover the ratio of fx to fS. Since the sampling clock frequency fS is a known value, the frequency fx of the input signal x(t) can be determined from the recovered frequency ratio.
While the FDC 100 in FIG. 1 is desirable in that it provides an all-digital solution, the logic gates used to implement the FDC have inherent limits on the speed at which they may operate. Consequently, the FDC 100 is not suitable for determining frequencies of very high frequency signals. There is a need, therefore, for digital circuits and methods that are capable of sampling and determining frequencies of high-frequency digital signals, and which are not limited by inherent speed constraints of the underlying digital circuitry used to perform the sampling.